When trying to transmit an information bit string (data object) to a receiver side in a DS-CDMA mode (Direct Sequence Code Division Multiple Access) used in mobile communication, the information bit string is modulated into spread spectrum signals by spreading the string with spread codes, and the modulated signals in a form of radio signals are transmitted to the receiver side for achieving communication.
Chip rate of the spread codes used in the DS-CDMA mode is a sufficiently high rate for a signal rate of the information bit string, and it is selected as a sufficiently high rate for propagation delay time between a mobile terminal device and a base station.
Therefore, it becomes possible to separate the radio signals reached via a specific propagation path based on the propagation delay time from the multipath signals that reach a mobile terminal device (receiver) via different propagation paths generated due to indirect reflected waves, diffracted waves, or the like, i.e., via different propagation path lengths. Each of the separated DS-CDMA signals having different propagation delay time is referred to as a multipath signal. It is well-known that a path diversity effect can be obtained and receiver gain can be improved through performing demodulation by allocating respective finger receiver units to the separated multipath signals and performing rake combining of each demodulated signal.
Separation of the multipath signals described above is performed based on a delay profile that shows received signal power distribution for the propagation delay time (reaching time to the receiver). The delay profile can normally be obtained through calculating the code correlations with respect to the spread codes of receiving waves by using a matched filter or a sliding correlator by each chip rate or by each over-sampling rate that is an integral multiple of the chip rate. FIG. 10 shows an example where there are three paths in the delay profile. The horizontal axis in FIG. 10 represents relative propagation delay time, and the longitudinal axis represents signal powers. Reference numeral 301 in FIG. 10 indicates a noise level, and it is assumed that there exists an effective path when the signal power is at a peak position 302 for the noise level 301.
For such paths present in the delay profile, the aforementioned finger receiver is allocated and demodulation is performed. When the allocation is done once, there is taken a measure to prevent allocation of other finger receivers to the vicinity of the allocated path, i.e., there is taken a measure to set a mask region to the vicinity of the path to which the finger receiver is allocated so as to prevent allocation of other finger receivers to the path to which the mask region is set.
Reasons for taking the above-described preventing measure are as follows. That is, paths in the vicinity of the significant path in many cases are side lobes of that path, i.e., paths that are not independent of the allocated path. Thus, even if finger receivers are allocated to such paths and rake combine is performed thereon, the path diversity effect cannot be obtained. In addition, noise and interference components are superimposed on the demodulated signals, which may possibly result in deteriorating the reception qualities. Furthermore, limited resources of finger receivers are to be used wastefully.
In addition to setting the aforementioned mask regions, state of the propagation paths is constantly fluctuated in the mobile telecommunications, i.e., there is a fluctuation occurred in the delay profile constantly. In order to maintain a fine reception quality, the delay profile is updated periodically. If it becomes necessary as a result thereof, there is performed tracking processing for the path whose propagation delay is fluctuated, or there is performed update processing of the allocation of the finger receivers for emerging paths and fading paths. For that, as shown in FIG. 11, employed is a method which performs the path tracking processing and the update processing of path allocation to the finger receivers through setting a path tracking region 303 for searching fluctuation of signal power peak positions, and mask regions 304 for preventing allocation of other finger receivers to the vicinity of the path that has been allocated to a finger receiver on the delay profile.
Patent document 1 discloses a synchronous tracking method in a spread spectrum communication system having a path search function. A delay profile measuring device, an extracting device for extracting path timing signals from the delay profile, and a path identifying device for identifying a subordinate path group and an independent path are used for the path tracking operation. The path identifying device contains a threshold value setting device that is capable of setting a threshold value, and identifies each path with the path timing signal that has a value equals to or higher than the threshold value (e.g., each path with the path timing signals within ±1-chip time length) as a subordinate path group.    Patent Document 1: Japanese Unexamined Patent Publication 2001-313590
There are also following issues even if the aforementioned known method is employed. That is, in a case of the delay profile shown in FIG. 12, i.e., a delay profile where three paths 305, 306, and 307 are close to each other (a delay profile especially with a small difference in the propagation delay time), the side lobes in the vicinity of the signal power peaks of the three paths 305, 306, and 307 overlap with each other, and there are the mutually overlapped signal power peaks of the three paths 305, 306, and 307 present in the delay profile.
When the aforementioned path tracking regions and mask regions are set for the paths 305, 306, and 307 in the delay profile shown in FIG. 12, a part of the path tracking region set for a single path comes to lose the function as the path tracking region because of the mask region set for its neighboring path as shown in FIG. 12, since the path tracking regions and the mask regions set for each of the aforementioned three paths 305, 306, and 307 overlap with each other.
Therefore, path tracking operations cannot be done in the right direction for the path 305, in both directions for the path 306, and in the left direction for the path 307. When such incapability of path tracking operations occurs continuously in fluctuation of the delay profile, in particular, the reception quality becomes deteriorated. At last, it may result in disconnection of calls.
In Patent Document 1, for identifying whether the paths in the delay profile are subordinate path group or independent path, it is judged whether the paths are the subordinate group or not only depending on the presence of the path timing signals but no other factors are considered for grouping the paths. That is, it is not designed to perform grouping of the paths other than the aforementioned manner.